Method and arrangement for magnetic digital recording with high frequency biasing

ABSTRACT

The invention relates to a method and circuit arrangement for magnetic recording of a digital data signal composed of a set of transitions. A recording signal is formed by superposition on the data signal of a high frequency, constant amplitude, magnetic biasing signal S p . The biasing signal is phase-modulated relative to the data signal transitions. The invention remedies &#34;peak shift&#34; and can be applied to any rectangular or sinusoidal biasing signal.

This application is a continuation of application Ser. No. 156,368,filed June 3, 1980.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method and an arrangement for magneticdigital recording with high frequency biasing.

2. Description of the Prior Art

The magnetic recording of a data signal consists basically in creating awrite current in a magnetic head, which produces on one face of amagnetic support such as a tape, disc or drum a residual magnetisationrepresentative of the initial data.

In the case of an electric analog data signal, for example a signalwhose amplitude is representative of sound from an acoustic transducer,it is evident that the residual magnetisation created by the writecurrent should faithfully respect, in time, the amplitude of the datasignal. This is universally obtained by superimposing on the data signala high frequency, constant amplitude, alternating current signal whichconstitutes a magnetic biasing. Put briefly, the curve of firstmagnetisation of the magnetic material of the recording carrier startswith a quadratically curving part, extends in a linear part and ends ina part which curves in to end in saturation. Without the magneticbiasing current, the recording of the data signal would correspond tomaking a more or less large excursion in the curve of the firstmagnetisation from its origin and at each point of the recording carrieraccording to the level of amplitude of the write signal. Thequadratically increasing part of the curve would always be involved sothat the recorded signal would present a high rate of distortion. Thesuperposition on the data signal of a high frequency, constantamplitude, alternating current signal constitutes a magnetic biasing inthe sense that the excursions can be made in the linear part of thecurve of the first magnetisation. Furthermore, the high frequencymagnetic biasing leaves the recording carrier non-magnetised in theabsence of the data signal, the more so since the length of thecorresponding wave of the biasing current is inferior to the power ofresolution of the read head, which power is essentially dependent on thevalue of the read air gap. On the other hand, although a direct currentmagnetic biasing is also possible, it would place the recorded carrier,in the absence of the data signal, in a permanently magnetised statewhich would have repercussions in the read signal in a great backgroundnoise.

A binary coded digital data signal indicates successively, at a givenrecurrence frequency, the value 0 or 1 of a data bit. This signalcomprises, therefore, two correlative components--a repeated series ofinstants and a series of corresponding binary values--which the residualmagnetisation should normally translate faithfully. To do this, it hasbeen attempted to represent at least one of the two binary values by amagnetic flux transmission determined as a function of a selected codeoccurring at a precise corresponding instant.

In current practice, magnetic transitions are advantageously inversionsof biasing of the residual field, designed to make this field changebetween two predetermined positive and negative biasing levels of themagnetic material of the recording carrier. The result of this is tocreate in this carrier a set of magnets placed end-to-end, with adjacentpoles of the same kind, and of length corresponding to the time intervalseparating two transitions conforming to the method of coding chosen. Byconvention, a reversal of the residual field from a negative level to apositive level of biasing will be called a positive transition, areversal in the opposite direction being a negative transition.

Among the types of coding most used is that called NRZ1 (non return tozero for bits of value "1"). In the NRZ1 code, only bits with a value of1 are represented by magnetic transitions independently of the directionof these transitions. In another popular code called "coded phase,"wherein the two binary values correspond respectively to the positiveand negative transitions. As will be seen later, the method of codingchosen does not matter for the purpose of the present invention.

Various problems are, in effect, related to the accuracy of recordingand reading of the other component of the digital data signal relativeto the instants at which the transitions should have taken place.

It has been noted previously that the binary data is translated on therecording carrier as a series of magnets placed end-to-end, of which theadjacent poles are of the same kind and translate the existence of atransition. The read current produced by the read head during passage oftwo adjacent semi-magnets is, therefore, in the form of a clock curve,of which the peak corresponds to the transition, since the variation inmagnetic flux in the read winding is greatest during passage of the twoneighbouring poles of the two magnets before the air gap of the readhead. However, when two transitions are very close together (which isthe case with high recording densities), the successive curves run intoeach other or combine so that the current peaks are offset from theactual transitions. This phenomenon, more generally known as peak shift,increases with the frequency of transitions so that, for high recordingdensities, the peaks can be shifted by up to about one third of thesmallest space which can separate two transitions. The decoding circuitsmust therefore be very active, the more so since to this shiftingvariations in the speed of travel of the recording carrier are added.Various attempts have therefore been made with a view to reducing thesize of the peak shift.

Results have been obtained in this direction by using a digitalrecording signal similar to an analogue recording signal. Experience hasin fact shown a reduction in peak shift for high write densities, abovearound 200 inversions of flux per millimeter, with a composite recordingsignal formed by the superposition of a high frequency, constantamplitude, magnetic biasing alternating signal on the digital coded datasignal.

In this composite recording signal, each transition is represented by adifference in peak amplitudes of the same sign as two neighbouringhalf-waves of the biasing signal which are present respectively beforeand after the instant of transition corresponding to the data signal.Thus, the high frequency biasing is of interest from the moment whenthese two alternations are separated by a fixed time interval,theoretically corresponding to the period of the biasing signal andresulting in a suppression of the peak shift. However, in alternatingbiasing, digital recording devices of the prior art, this time intervalcan deviate unequally and erratically from the value of this period andcan cause uncertainties and errors in decoding the signal registered bythese devices. These deviations result from the distribution oftransitions in the coded digital data signal, the latter being thereforeable to arise at any instant in a period of the magnetic biasing signaland act so that the superposition of the two signals is more or lessfavorable. The more favorable situation (zero deviation) occurs whenthere is a coincidence between a transition of a given sign and the peakamplitude of the same sign of the high frequency biasing signal. On theother hand, the deviation is maximum when the transition of a given signoccurs at the moment when the biasing signal reaches a peak amplitude ofthe opposite sign, in which case the following peak amplitude is delayedby about half a wave-length of the biasing signal.

It follows that the size of the peak shift depends on the phase of themagnetic biasing signal with respect to the coded signal and that, ifthe peak shift is on average effectively reduced by the alternatingbiasing, relatively high values can be obtained for certain transitionsand very active circuits will be needed for the reading and decoding ofsignals recorded in this manner.

To avoid this peak shift, it would appear of interest to render thebiasing signal synchronous, as regards frequency, with the clock forcontrolling the coded digital signal to be recorded. However, becausethe positive and negative transitions in the coded signal aredistributed in a random manner, the phase that exists between eachtransition and the magnetic biasing signal remains uncertain, so thatmore or less favorable cases will still occur, as in the preceding case.

It should be apparent, on the other hand, that the increase in thefrequency of the biasing signal with respect to the higher frequency ofrecurrence of transitions diminishes the peak shift effect. Also, theincrease in the frequency of the biasing signal is in practice quicklylimited by the fact that it raises the electromagnetic losses in thematerials forming the recording heads as a result. Furthermore, anattenuation of the peak shift is observed when there is a judiciousrelationship between the frequency of the magnetic biasing signal andthe clock frequency according to the rhythm at which the coding iseffected. Nevertheless, there remain unfavorable cases which could stillcause alteration of the restored message in certain cases and, as aresult, necessitate the presence of a sufficiently improved read anddecoding device to remove these risks.

SUMMARY OF THE INVENTION

The invention remedies the existence of any unfavorable case for adigital magnetic recording with high frequency biasing.

To this end, the invention relates to a method of magnetic recording ofa digital data signal composed of a series of transitions, of the typeconsisting in superimposing a high frequency, constant amplitude,magnetic biasing signal on the data signal, and is characterized in thatthe phase of the biasing signal is modulated on the data signaltransitions.

In addition, the invention relates to an arrangement for magneticallyrecording a digital data signal composed of a series of transitionscomprising magnetic biasing means for delivering an original highfrequency, constant amplitude, biasing signal, and means for combiningthe data signal with the magnetic biasing signal so as to provide arecording signal, and characterized by means for modulation of the phaseof the original biasing signal relative to the data signal transitions.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics and advantages of the invention will be seen moreclearly from the text which follows, with reference to the accompanyingdrawings, in which:

FIG. 1 illustrates an example of magnetic recording in NZR1 code of theprior art technique, not using alternating magnetic biasing;

FIG. 2 comprising parts 2A and 2B illustrate, respectively, the mostfavorable and least favorable cases of digital recording withalternating biasing according to the prior art;

FIG. 3 illustrates an example of digital magnetic recording in NZR1 codewith alternating biasing according to the invention;

FIG. 4 shows an embodiment according to the invention of a digitalrecording arrangement with alternating biasing;

FIG. 5 comprising parts A to F illustrate examples of wave-forms anddata which can be obtained at various points of the recordingarrangement shown in FIG. 4;

FIG. 6 shows a first embodiment according to the invention of a digitalrecording arrangement with alternating biasing;

FIG. 7 comprising parts A to G, E', H and F' illustrate examples ofwave-forms and data which can be obtained at various points of therecording arrangement shown in FIG. 6;

FIG. 8 shows, in broken lines, part of the theoretical signal shown inFIG. 7F' and indicates, by a solid line, how this signal is made inpractice;

FIG. 9 illustrates a variation according to the invention of the signalshown in FIG. 7F';

FIG. 10 shows a second embodiment according to the invention of adigital magnetic recording arrangement with alternating bias and;

FIG. 11 comprising parts A-J and E' illustrate, by way of example,waveforms and data which can be obtained at various points of therecording device shown in FIG. 10.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be better brought out following a short descriptionof the results obtained by previous digital magnetic recording devices,with reference to FIGS. 1, 2A and 2B.

FIG. 1 relates to a digital magnetic recording of the NRZ1 type with nobiasing. In this Figure, I_(o) designates the original binary data to berecorded, formed by a recurrent set of bits such as are indicated by wayof example. S_(c) designates the recording signal, also called the writesignal, resulting from the coding of the original data I_(o) in NRZ1.S_(l) designates the corresponding read signal obtained across theterminals of the winding of the read head on passage of the data carriercarrying the recording of the write signal S_(c). S_(r) designates thesignal reproduced from the read signal S_(l) with a view to obtaining arepresentation of the digital signal read in the NRZ1 code, and I_(r)designates the information reproduced after decoding signal S_(r).

Thus the recording signal S_(e) is an alternating signal of which thetransitions correspond to the bits of value 1 and of which the positiveamplitude +n and the negative amplitude -n ordinarily correspond to thepositive and negative saturation levels of the magnetic recordingmaterial. A read signal, obtained from the terminals of the winding of aread head, has the shape of a positive or negative humped curveaccording to whether the transition itself is positive or negative andthe summit (or peak) of which represents the moment of transition. Theoccurrence of the peak should correspond to the recording of atransition which is present and isolated on a data carrier. However,because of the variable proximity of the transitions on the one hand andof the high density of recording sought for on the other hand, thehump-shaped characteristics produced at each transition combine togethermore or less according to their proximity and, as a result, have peaks+p and -p more or less offset from the representative instants of thetransitions. Various peak shift values d0, d1, d2 and d3 are indicatedby way of example in FIG. 1. Given that the peaks determine the instantsof appearance of the transitions and that they are variably offset fromthese transitions, the signal S_(r) reproduced by the peaks of the readsignal S_(l) is different from the recording signal S_(e) although,ideally, the signals S_(e) and S_(r) should be identical. Because of thepeak shifts d0 to d3, the decoding of the reproduced signal S_(r)results in an unfaithful reproduction which can produce an item of dataI_(r) which is different from the original item of data I_(o). In theexample shown, the numbers in broken lines illustrate the errors whichcan be made in the reproduced signals I_(r) due to decoding of thesignal S_(r).

FIGS. 2A and 2B relate to a digital recording using the magnetic biasingaccording to prior art. In these figures; S_(ic) designates a coded datasignal produced from an original item of data (now shown) according toany code and alternating between two predetermined levels, references +1and -1. S_(p) designates an alternating, magnetic bias signal at highfrequency and having constant amplitude c±. S_(e) designates therecording signal resulting from the superimposition of the precedingsignals S_(ic) and S_(p), and S_(r) designates the signal reproducedafter recording and reading of the signal S_(e). From FIG. 2A, themoment t_(o) of appearance of a positive transition of the data signalS_(ic) corresponds to the most favorable recording situation, while inFIG. 2B, the moment t_(l) corresponds to the least favorable situation.

In these figures the recording signal S_(e), which excites the windingof the write head, is an oscillating alternating current on both sidesof a transition, between values +a to -b and -a to +b. Intensities of +arelate to weak currents and intensities of +b represent strong currentsacross the winding. The positive and negative transitions arerespectively represented by jumps between the levels +a to +b and -a to-b. Detection of the peak of the first half-wave which marks such ajump, is interpreted after reading the recorded signal S_(e), as themoment of transition as illustrated by the signals S_(r) reproduced inFIGS. 2A and 2B. Under these conditions, on the one hand the linearityof the alternating signal S_(e) means little (contrary to the analoguerecording) so that the +b levels correspond in practice to thesaturation levels of the magnetic material of the recording carrier andso that the relationship between the reference level 1 of the datasignal S_(ic) and the level c of the biasing signal S_(p) is relativelylarge (generally of the order of 1/4) compared with that generally used(1/10) in analogue recording. On the other hand, it is desirable thatthe peak of the first positive or negative half-wave marking thetransition in recording signal S_(e) coincides respectively with thecorresponding positive or negative transition of the data signal S_(ic)so as thus to avoid any shifting of transitions in the signal S_(r)reproduced. However, due to the fact that the transitions of the datasignal S_(ic) are distributed randomly in time, more or less favorablecombinations with the alternating biasing signal S_(p) will be produced.The most favorable situation is shown in FIG. 2A, from which it is seenthat the positive transition at the moment t_(o) of data signal S_(ic)coincides with a positive peak of the biasing signal S_(p) and that thusthe superposition keeps this half-wave unchanged in time in order todesignate the transition. The same applies for a negative transition ofthe data signal S_(ic) and a negative peak of the biasing signal S_(p).The least favorable situation is described in FIG. 2B, in which thepositive transition at the moment t₁ of data signal S_(ic) coincideswith a negative peak of the biasing signal S_(p) and the superpositionintroduces a delay d' in the appearance of the first half-wavetransition in the recording signal. This delay is equivalent to ahalf-period of the biasing signal S_(p). The same would be the case fora negative transition conjoint with a positive peak. The peak shift d'therefore relates the transition to a moment t₂ which, with highrecording densities, could cause alteration of the contents of theoriginal item of data.

By comparison with FIGS. 2A and 2B, FIG. 3 shows the advantages of adigital recording with biasing according to the invention. In FIG. 3,I_(o) designates an original binary item of data. S_(ic) designates thecorresponding data signal coded in NRZ1. S_(p) designates an originalbiasing signal which is directly used in previous recording devices.S_(pi) designates a biasing signal according to the invention, and S_(e)designates the recording signal resulting from the combination ofsignals S_(ic) and S_(pi). It is apparent from the shape of the wave ofthe biasing signal S_(pi) that, according to the invention, the originalbiasing signal S_(p) is phase-modulated on the transitions of the codeddata signal S_(ic) in the sense that, from each of these transitions theoriginal biasing signal S_(p) undergoes a successive phase-shift of180°. In this way, the biasing signal S_(pi) of the invention comprisesextended high levels n and low levels l which, combined with therespective transitions of the data signal S S_(ic) are able to reproducethe high and low levels in the recording signal S_(e) in conditionswhich are always favorable without any shifting.

FIG. 4 shows an embodiment of a magnetic digital recording arrangement10 according to the invention, which will be described with reference toFIG. 5 illustrating, by way of example, waveforms A-F which can beobtained at various points of the circuit arrangement 10.

The digital recording device 10 according to the invention incorporatesa clock 11 which delivers a clock signal 5A composed of a series ofpulses recurring at a given frequency f_(H) and at a 0.5 cycle ratio(ratio between the duration of each pulse and their period ofrecurrence). Signal 5A is applied to an input of a -N frequency divider12, which produces a signal 5B composed of a series of recurring pulsesf_(H) /N (N being a whole number whose value here is 3). An encoder 13has a first input receiving the output signal 5B of the frequencydivider 12 and a second input receiving an original item of data 5C tobe recorded. According to the example shown, encoder 13 extracts fromsignal 5C the transitions at the rate of the signal 5B and effects apredetermined coding to deliver a coded data signal 5D equivalent to theabove-mentioned signals S_(ic). The code chosen by way of example inFIGS. 4 and 5, as in the other figures which follow, is such that thetransitions are representative of the bits of value 1 of data 5C so asto obtain a recording signal of the type NRZ1. It will be noted that thesignals 5A, 5B and 5D shown and the signal 5E, which will be consideredfurther on, are direct current signals of logic values 0 and 1, althoughthese signals could be alternating like those shown in the precedingfigures. Actually, the components introduced into the circuitarrangement 10 for the formation of these signals operate on directcurrent, a conversion into alternating current of the signals concernedfor the formation of the recording signal being made later.

The clock signal 5A and the data signal 5D are applied respectively totwo inputs of an exclusive OR gate 14 which delivers a signal 5E at itsoutput. The gate 14 plays the part of the phase modulator modulating, ata predetermined value (180°), the phase of the clock signal 5A(equivalent to the original biasing signal S_(p) indicated in FIG. 3) oneach appearance of the data signal transitions 5D to furnish a modulatedbiasing signal 5E equivalent to the signal S_(pi) of FIG. 3.

The biasing signal 5E coming from the gate 14 and the coded data signal5D are fed respectively two inputs of a combining circuit element 15,which furnishes at its output a recording signal 5F intended for therecording winding 16 of the recording head, which is not shown. Thecombination circuit 15 is formed basically by two current switches 15a,15b which handle respectively the biasing signal 5E and the data signal5D. The switches 15a and 15b are respectively formed of two currentsources 17a and 17b, which are supplied by a voltage source +V, and oftwo pairs of transistors 18a, 18a', 18b and 18b', of which the emittersare connected in common to the output of the respective current sources17a and 17b and the collectors are connected to the end terminals of therecording winding 16. The center point of winding 16 is connected to apredetermined voltage -V.

The biasing signal 5E delivered by the gate 14 is applied directly tothe base of the transistor 18a and, by means of an inverter 19a, to thebase of the transistor 18'a. In the same way the data signal 5D isapplied directly to the base of the transistor 18b and, by means of aninverter 19b, to the base of the transistor 18'b. The switches 15a and15b thus switch the currents corresponding to signals 5D and 5E, whichare thus made alternating. These currents are added together in thewinding 16 in the form of signal 5F similar to the recording signalS_(e) shown in FIG. 3. It will be noted that in FIG. 5, due to the phasedisplacement by 180° which occurs in the signal 5E on each transition ofthe data signal 5D, this phase modulation produces transitions from +ato +b and from -a to -b in the resulting signal 5F at instants whichalways correspond to the data signal transitions.

In the example shown in FIGS. 4 and 5, the clock signal 5A controls theformation of the data signal 5D by means of the frequency divider 12.The magnetic biasing signal 5E is synchronized with the clock signal 5Adue to the phase modulator constituted by the gate 14.

FIGS. 6 and 7 show a circuit arrangement 20 forming a modification ofthe embodiment of the digital recording arrangement 10 which has justbeen described. The similarity between the arrangements 10 and 20 isexplained by the fact that the elements 21 to 28, 28' of the arrangement20 correspond respectively to elements 11 to 18, 18' of the arrangement10. Furthermore, signals 7A to 7E are similar respectively to signals 5Ato 5E, while the signal 7E' represents a version of the signal 7Eshifted in time and the signal 7F' represents the signal resulting fromsignal 7E' and also forms a version of the signal 5F which is shifted inthe same time. The signals 7G, 7H are new signals involved in theembodiment shown in FIG. 6.

More precisely in the same way as the recording arrangement 10, thearrangement 20 comprises a clock 21, a frequency divider 22, and encoder23, an exclusive OR gate 24 and a combining circuit element 25 supplyinga recording winding 26 and including a current source 27 which suppliesa pair of transistors 28, 28' which are intended for excitation of thewinding 26. The clock 21 furnishes a clock signal 7A to a frequencydivider 22 and to an input of the gate 24. The divider 22 divides thefrequency of the clock signal 7A by a predetermined whole number N (hereN=6) to form a signal 7B acting as a clock for coding the original datasignal 7C entering the encoder 23. This encoder effects the frequencysynchronization of signals 7B and 7C and thus furnishes the other inputof gate 24 with a continuous coded signal 7D. Gate 24 constitutes aphase modulator to form a modulated biasing signal 7E, in which eachtransition of the data signal 7D successively triggers a phasedisplacement by 180° of the clock signal 7A.

The embodiment shown in FIG. 6 activates a conventional unit 25 which isused as an element for excitation of the coil 26. The unit 25 iscomposed of two transistors 28 and 28' and the current source 27 whichhas two inputs 27a and 27b for the control of strong and weak currentsrespectively. The example shown relates to control by the input 27a, bywhich the weak current furnished during normal operation by the source27 is switched to a strong current of predetermined intensity during apulse of the signal applied on the terminal 27a. To effect this controlfrom the clock signal 7A, the combination element 25 comprises a Dflip-flop 29. A D flip-flop has a data input D (Data), a control input G(Gating), and two outputs, direct Q and complementary Q. In a flip-flopof this type, the output follows the input on the order of the controlsignal applied to the input G. As a result, the flip-flop 29 receivesthe clock signal 7A on the input D and on its control input G a doublefrequency signal 7G from the output Q of an astable flip-flopconstituting a clock 30. For convenience, the direct output Q of theclock 30 feeds a divider by two circuit which forms the clock 21 andwhich furnishes the square signal 7A (which has a cyclic ratio of 0.5).

Under the conditions shown in FIG. 7, the output signal of the Dflip-flop 29 takes the shape of the signal 7H. As the signal 7H isshifted by a quarter of a period from the biasing signal 7E, a Dflip-flop 31 is inserted between the output of the gate 24 and thecurrent switch 28, 28' to effect the same shift in the modulated biasingsignal 7E. Thus the signal 7E from the gate 24 is applied to the D inputof flip-flop 31, the latter being controlled by the signal 7G comingfrom the output Q of the clock 30. The direct output Q of the flip-flop31 excites the base of the transistor 28' through the amplifier 32,while the traverse output Q excites the transistor base 28 by means ofthe amplifier 33. In this way the signal circulating in the recordingwinding 26 is of the form shown at 7F', this signal being a versionwhich is shifted by a predetermined time interval (here a quarter of aperiod) of the transitions of the data signal 7D. As this time period isconstant, the signal 7F' corresponds to the signal 5F relative to thebasic device shown in FIG. 4.

This system has the advantage of obtaining a precise phase relationshipbetween the strong current control signal 7H and the current switch (28,28') control signal 7E' due to the intervention of the clock signal 7Gcontrolling the two flip-flops 29 and 31 which generated signal 7H and7E'. The device 20 is, as a result, not affected by the wave propagationtimes in the various components which precede the flip-flops.

Another advantage of the invention is shown in FIG. 8 which shows thetheoretical signal 7F' as a fine line and the shape 7F" of the signalobtained in practice on the recording support as a solid line. It willbe noted, in effect, that the half-wave peak amplitude which marks eachtransition is greater than that of the other half-wave. Taking intoaccount the resistive and capacitive parasitic components of therecording winding, the latter introduces a time constant forestablishment of the recording signal about the peak values +a and +b.The result is that the actual signal 7F" cannot normally reach thesevalues in the space of a half period and breaks at peak values |a'|<|a|and |b'|<|b|. Because of the levels h and l introduced into themodulated biasing signal according to the invention, the actual signal7F" has a whole period available so that the peak amplitudes |b"| of thehalf-waves marking the transitions are practically equal to |b|. Thisincreases the reliability of reproduction of the recorded message.

It will also be noted that the positive and negative transitions of thedata signals 5D and 7D correspond respectively to the low levels l andhigh levels h in the recorded signals 7F and 7F'. FIG. 9 shows arecording signal 7F"' which would be obtained in place of the signal 7F'if the output Q of the flip-flop 29 (FIG. 6) fed the input 27b whichcontrols the weak currents from the current source 27 and the connection19 Q-27a was suppressed. In this case it is seen that the positive andnegative transitions of the data signals 5D and 7D correspondrespectively to the high and low levels of the recording signal 7F"'.

It will finally be noted that, in a general manner, in a circuitarrangement 20 such as illustrated in FIG. 6, the frequency of thebiasing signal 7E should be a whole multiple (here 2) of the frequencyof the signal 7G delivered by the clock 30.

FIGS. 10 and 11 relate to a magnetic recording arrangement 40 which is amodification of the embodiment of the arrangement 20 which has just beendescribed with reference to FIGS. 6 to 9. The similarity between thearrangements 20 and 40 is seen from analogy in numbering the componentsand signals, the components 41 to 53 of the arrangement 40 correspondingrespectively to components 21 to 33 of the arrangement 20 and thesignals 11A to 11H corresponding respectively to signals 7A to 7H. Infact, the difference in structure of the arrangements 20 and 40 residesin the formation of the clock signals for the gate 44 and thecombination element 45. Whereas, in the arrangements 10 and 20, theclock signals 5B and 7B, which control the coding of the data signalwhich enters the respective coders 13 and 23, come from a clock 11 or 21by frequency division, in arrangement 40 the signal 11B which serves forcoding the original in information 11C in encoder 43 is directlyproduced by a base clock 54 from which is formed the clock signal 11A bysuccessive modifications in frequency. Thus the signal 11B passes into adivide-by-two circuit 55 which delivers a signal 11I which is applied toa first input of a phase comparator 56. This comparator is the firstelement of a phase-locking system, which further comprises elements 50,41, 42 and 57, and operates in such a way that its output signalcorresponds to the signal 11I to control the astable flip-flop 50. Thefrequency of the signal 11G is thus in a predetermined relationship withsignal 11I just as the signal produced at the output Q of the oscillator50 which controls the clock 41. This clock is a divide-by-two circuit soas to furnish the clock signal 11A which is necessary for theelaboration of the biasing signal 11E which comes from the gate 44. Theclock signal 11A is frequency-regulated by a return loop formed by adivider 42 which divides by N (N=6 in the example shown) followed by adivide-by-two circuit 57 producing a signal 11J applied to a secondinput of the phase comparator 56. In this way any variation in phase ofthe signal 11J is corrected by the comparator 56 to tend towards that ofthe signal 11I.

Although the examples shown are based on the NRZ1 coding, the precedingtext brings out clearly that the invention can be applied to any sort ofcoding involving any set of transitions whatsoever (such as coded phasecoding for example). In other words, the invention is in no way limitedto the embodiments which have been described and illustrated but, on thecontrary, comprises all technical equivalents of the means described inthe claims which follow. On the other hand, although the clock signalsrepresented are square pulse signals (with a cyclic ratio of 0.5), theinvention is applicable equally to any rectangular pulse signal (of anycyclic ratio) or any alternating wave signal such as a sinusoidalsignal, for example. In effect, only strong and weak current intensitiesare taken into account in the reading and reproduction of data sincethey determine the work zone on the magnetisation curve of the materialindifferently to the switching times between strong and weak currents.Furthermore, due to the phase modulation of the biasing signal accordingto the invention, it has been seen that the combination of this codeddata signal always occurs favorably.

What is claimed is:
 1. A circuit arrangement for magnetic recording of adigital data signal composed of a set of transitions comprising magneticbiasing means for delivering a high frequency, constant amplitude,original biasing signal and means for combining the said digital datasignal with the said magnetic biasing signal so as to provide arecording signal, said combining means comprising a current switchdevice including a first and a second current switch connected toreceive, respectively, complementary formats of the data signal, arecording winding connected to said first and second switches to producethe recording signal, and a current source for each said current switchsupplying the associated current switch and means for phase modulatingthe original biasing signal relative to the data signal transitions. 2.A circuit arrangement for magnetic recording of a digital data signalcomposed of a set of transitions comprising magnetic biasing means fordelivering a high frequency, constant amplitude, original biasingsignal, said biasing means comprising a source of clock signals having afirst version used for the formation of the said original biasing signaland a second version used for the formation of the said data signal, andmeans for combining the said digital data signal with the said magneticbiasing signal so as to provide a recording signal, said combining meanscomprising a current switch device including a first and a secondcurrent switch connected to receive, respectively, complementary formatsof the data signal, a recording winding connected to said first andsecond switches to produce the recording signal, and a first and asecond current source for each said current switch supplying theassociated current switch and means for phase modulating the originalbiasing signal relative to the data signal transitions.